The invention relates to the field of infrared sensors in which infrared radiation is detected with an array of photodetectors, converted to electrical signals and then electronically displayed, recorded or used for automatic detection and/or track of targets. Major fields of use include night vision devices such as the forward looking infrared (FLIR) systems, surveillance systems, and sensor/tracking systems for weapon control. The invention finds greatest utility in detecting extremely weak objects or objects whose emitted radiation has been strongly attenuated by atmospheric conditions and in using detector arrays with a substantial number of defective elements. Prior art systems are known to use two-dimensional arrays and scanners along with coolers for detectors and focusing optics. These are the common building blocks of modern infrared sensors. Pat. No. 4,806,761 to Carson et al. employs a two-dimensional detector array which is carried by and mounted on the focal plane of an optical/electronics module which has embedded in it amplifying, filtering and multiplexing circuitry utilizing MOSFET transistors. Carson et al. suggests use of a limited nutation scanning mode to modulate the incoming radiation and to allow comparison and calibration to remove the low frequency (1/f) noise from the lead selenide photodetectors. The main thrust of the Carson et al. device is to achieve acceptable performance out of inexpensive detector materials by using Z-technology electronics packaging and signal chopping/scanning to solve problems of excessive low frequency (1/f) noise and overcome the inherent low sensitivity of lead selenide photodetectors.
Another prior art Pat. No. 4,728,804 to Norsworthy employs a scanner and multiplexer to reduce processor complexity and the number of electrical connections required between the elements of the sensor.
Another prior art Pat. No. 4,767,9374 to Norsworthy employs a scanner and special detector array geometry to permit a high frame rate with reduced target scan rate, reduced processor complexity as well as reducing the number of electrical connections required between the elements of the sensor.
Problems associated with the patents discussed above and the prior art in general is an inability to fully benefit from the use of high density two-dimensional arrays of the most sensitive photodetector materials such as HgCdTe, InSb, etc. Prior art devices using high density two-dimensional arrays of highly sensitive detector materials ar limited by the spatial noise or residual detector nonuniformity which remains even after array calibration and normal electronic correction. Prior art devices also suffer from an inability to remove the effect of dead or totally nonfunctioning detector elements. Although some cosmetic improvement to the images may be made by electronics, missing information is still missing.
It is very difficult or costly to build large two-dimensional arrays of detectors from some materials where all the detectors have an acceptable responsivity and noise level. As a result, the majority of highly sensitive large format focal plane arrays have some detectors which are "dead" or "weak" or "noisy". "Dead" detectors have either no measurable responsivity or responsivity of an extremely low level. "Weak" detectors have measurable responsivity, but it is so low that an unacceptably noisy signal results after amplification. "Noisy" detectors may have normal responsivity but they have a noise level significantly greater than the array average. Even acceptable detector elements have a range of responsivities and hence produce a nonuniform display with spatial (fixed pattern) noise. Spatial noise from acceptable detectors can be (and normally is) partially corrected by a 2 (or sometimes 3) point linear correction process. However, such processes are inexact, and result in residual errors from several sources.
A major feature of Applicants' invention is the ability to use a photodetector array that otherwise would be unacceptable due to the number of defective or out of tolerance elements and still get a high level of performance. For instance, one faced with equipment design could spend the resources to use a photodetector array that was 99.9 percent defect free with associated problems and costs in attempting to obtain a near perfect array or could use Applicants' method and/or device to obtain superior results with an array that is only 95 percent defect free.
Thus, it is an object of this invention to reduce spatial noise resulting from residual compensation errors.
A further object is to teach a method and device to reduce or eliminate the effects of dead, weak or noisy photodetectors which view or scan an image point.
It is a further object to teach an Advanced IR Sensor which more fully benefits from large, high density, high quantum efficiency photodetector arrays made from such materials as HgCdTe and InSb.
Yet another object is to teach a method of obtaining high performance from a photodetector array with reduced costs.
It is still a further object of the present invention to teach a system which addresses the effects of spatial defects and nonuniformity which remain after gain and level correction.